Two basic methods of increasing photopolymer quantum yield beyond unity exist. The first of these is most familiar as the acrylate chemistry used in most commercial free radical U.V. cure systems. The approach here is that of the chain reaction. Any photon which is absorbed and becomes converted by the initiator moiety into a radical is capable of converting many polymerisable molecules very quickly. Thus the quantum yield for this process is high, but still not as high as that for silver halide.
The second basic form of photopolymer quantum yield enhancement is exemplified by the cationic U.V. curing systems. In this instance, the absorbed photon generates a catalytic monomer species which is capable of catalysing polymerisations, cross-linking, or even molecular cleavage. This technology has been described as capable of producing “living polymers” which will continue growing as long as substrate monomer molecules are still available. The reactions are, however, relatively slow compared to the chain reactions of the free radical process. Furthermore, although the quantum yield in terms of reacted molecules is theoretically near infinite, the slow reactions limit spatial resolution by reason of diffusion of active species out of the imaged area.